In this article, a refiner needed a
solution to recover export-oriented naphtha contaminated by methanol (MeOH). This is an actual
case in which one of three 30,000-m3 capacity tanks
used to store naphtha export was found to be severely
contaminated with MeOH. A water-washing solution was applied to
reprocess the naphtha in-situ and to remove MeOH from the
naphtha, along with returning the storage tank back to
continuous operations.

Problem-solving
approach. The approach to solving problems such as
this type is threefold. First, understand the
root cause of the incident and rectify it.
Second, develop a theoretical basis to resolve
the problem and validate it. Third, evaluate
and implement a practical solution.

Contamination mishap.

Naphtha contamination with
oxygenates, such as MeOH, is a costly problem for any refinery.
Reprocessing or re-blending naphtha is a risky proposition,
especially when only limited storage is available. A creative
and scientific water-washing solution was identified to remove
oxygenates from the naphtha. Understanding the chemistry of the
problem is only the first step. Substantial laboratory and
engineering work was necessary to successfully identify and to
validate the solution.

PROBLEM: MeOH CONTAMINATION
IN NAPHTHA TANK

This Middle East refinery exports straight-run
naphtha (SRN) from the crude distillation unit (CDU). There are
three storage tanks available to store, blend and certify the
naphtha prior to shipping. The refinery also operates a
tertiary amyl methyl ether unit that uses imported MeOH as a feedstock.

Following a routine MeOH unloading
at the refinery, 25,000 m3 of SRN product in an
export 30,000-m3 storage tank was later found to be
contaminated. This naphtha failed a product-certification test.
Contamination results were further confirmed by a third-party
laboratory.

Test results of the SRN showed an
oxygenate content of 240 ppm against the maximum acceptable
level of 50 ppm to obtain a quality certificate. Unfortunately,
the SRN of this tank could not be exported. Now, there were
only two naphtha tanks left in operation. The quantity of
material and oxygenates prevented meaningful re-blending and
reprocessing. Contamination of the other two tanks remained a
real and immediate threat to plant operations. During the
incident, naphtha rundown from the process plant was
continuously showing acceptable oxygenate content that was less
than 50 ppm.

Fig.
1. Water is a polar molecule with
positive charges on one side and negative
charges on the other.1

Step 1: Identify and rectify root cause.

The investigating team successfully
determined the root cause for the MeOH contamination. The
investigation revealed that both the meters installed on the
line to the jetty for naphtha export and the meter on the MeOH
import/unloading line share the same meter prover. A single
cross valve on the meter prover was mistakenly left open.
Pressure differential allowed imported MeOH from the vessel to
flow unimpeded to the naphtha-product line from the CDU to the
storage tank during unloading of the cargo vessel.

Rectification of the problem
involved strict new operating procedures for the meter and
meter prover. Operator training was improved, and it focused on
careful handling of equipment with checklists and verification
by foremen. For the long-term solution, a separate meter and
meter prover would be installed.

Different ways of disposing the
off-spec naphtha were studied. However, disposal could impose
higher financial losses to the company due to contamination of
the naphtha lines to the jetty. Moving the material to other
tanks had a higher inherent risk of cross-contamination for the
remaining product-storage tanks. At that time, no buyer could
be found to purchase this batch of off-spec naphtha.
Unfortunately, the MeOH-contaminated naphtha remained in the
tank for several months as various options were considered.

Step 2: Theory and validation.

The rule for determining if a
mixture becomes a solution is that polar molecules will mix to
form solutions and nonpolar molecules will form solutions, but
a polar and nonpolar combination will not form a solution. Both
MeOH and water are polar. So extraction of MeOH in an aqueous
solution is a feasible pathway. The geometry of the atoms in
polar molecules is such that one end of the molecule has a
positive electrical charge and the other side has a negative
charge. Nonpolar molecules do not have charges at their ends.
Mixing molecules of the same polarity usually results in the
molecules forming a solution.

Low-molecular-weight alcohols, such
as MeOH, are completely soluble in water. Because of their
polar structure, the alcohol molecules actively associate with
water molecules through the hydrogen bonds. The hydrogen bonds
are strong enough to prevent separation of the water/alcohol
mixture by distillation, as shown in Fig.
2.2

Fig.
2. MeOH hydrogen bonds and
polarity.2

Various molecules may mix and
dissolve in each other if they have approximately the same
polarity. In the case of water and MeOH, this is the situation.
The hydrogen of the OH group on the alcohol is polar in
the same manners as the water molecule.

Fig.
3. Lab results of water washing
of contaminated SRN.

Solubility of MeOH in naphtha.

In terms of polarity, MeOH is a
strong polar molecule, and aromatics, such as toluene, are
slightly polar. Paraffins, such as hexane, are nonpolar.
Aromatics will be temporarily polarized within the vicinity of
a polar molecule (MeOH), and the induced and permanent dipoles
will be mutually attracted (Debye Interactions). However, MeOH
is not completely soluble in streams, such as SRN that contain
low levels of aromatic compounds. Paraffinic/naphthenic
hydrocarbons (HCs) comprise 90 wt% of the SRN, and the
remaining 10% are aromatic HCs. Therefore, the MeOH and naphtha
are not soluble in any large ratios.

SRN, depending on the crude type
processed, normally contains 8 wt%10 wt% of aromatics.
MeOH solubility in aromatics is temperature dependent.
Essentially above 0°C, for every percentage of aromatics
present, 0.5% of MeOH will be soluble. Following this rule, it
is expected that the SRN can dissolve up to a maximum of 4
wt%5 wt% MeOH.

Laboratory testing was proposed and
arranged. Test samples with different water concentrations were
added to known volumes of the off-spec naphtha0% water
content in naphtha was the control sample with 1%, 5%, 10% and
20% water concentration standards tested. To investigate the
effect of thorough mixing, the samples were analyzed with and
without a magnetic stirrer used. Table 1 summarizes the lab
results.

Another set of tests was done on
the samples from the contaminated tank to measure the effect of
water washing at different vol% of water to remove the various
oxygenates from the contaminated naphtha. Test results show
that water washing removed the majority of the MeOH content
from the naphtha while other oxygenates were not affected.
Table 2 lists these test results at different water-wash
volumes with and without mixing. Fig. 4 shows the appearance of
the SRN after water washing at different vol% of water with a
one-hour settlement time and the settled water drained from the
sample. These tests showed that there was not much difference
in haziness of the naphtha when different volumes of wash water
were used. The lab report can be summarized as:
 MeOH and total oxygenate content decrease
dramatically to within specs (50-wppm maximum) when the
contaminated naphtha was water-washed with subsequent mixing
(by a magnetic stirrer similar to actual tank mixing). The MeOH
content remained high when mixing was not done.
 There was no change in color and the product was
not hazy.
 There is only a slight increase in water content
after the sample remains stagnant if water is not
drained.

Fig.
4. Haziness of treated naphtha after
water washing with different water
volumes.

The tests also confirmed the understanding that, if water
washing is done together with mixing, MeOH removal would be
more efficient. Based on these results and the lab report, it
was also decided that the contaminated naphtha should be washed
with demineralized (DM) water.

Lab results showed that 10% water
addition to the naphtha with mixing would reduce the MeOH
content from 190 ppm to 1.4 ppm, while 1% water can reduce it
from 190 ppm to 22.7 ppm. Subsequently, it was decided to
inject only 1% of water wash and to drain after mixing, and
repeat several times, until the total oxygenate concentration
dropped to less than 50 ppm. Using this method, less DM water
would be used, thus, limiting cleanup costs and time to recover
the product naphtha.

Fig.
5. Process scheme for Option 1:
Direct water injection to tank.

Step 3: Effective implementation.

Now that a lab-scale solution was
available, the emphasis shifted to execution. Several ideas
were considered, with three of the most viable choices listed
here:

Option 1. Direct water
injection to tank. Water can be pumped directly into
the tank T6217C. After injection, the SRN could be mixed with
the aid of an available tank mixer. Advantages of this process
were:
 Water can be introduced through a larger nozzle
(4-in. size).
 The associated lines would not be
contaminated.
 The procedure can be done several times. In case of
failure, the other two naphtha tanks would remain available for
rundown and dispatch.

The disadvantages included:
 Mixing will require a longer time.
 Mixing may not be as effective as circulating the
SRN to and from the tank.

Fig.
6. Tank mixing patterns for Option 1:
A and B.

The tentative time required for each cycle of water, assuming
1% DW will be mixed to the naphtha tank and drained after
mixing and settlement, are summarized Tables 3 and 4.

Option 2. Water injection via export piping.
Naphtha inventory of the tank can be circulated by way of
marine-loading pump. Water can be put into the suction line of
the pump0.75-in. nozzle with two nozzles with a capacity
of 3 m3/hr per nozzle. The resulting mixing could be
done by the pump itself and would not rely on the effectiveness
of the tank mixer. The advantages of this option include:
 There is thorough mixing of SRN and water
 The mixing time will be shorter
 The experiment can be carried out several times. In
case of failure, the other two tanks will be available for
rundown and dispatch.

However, the disadvantages
are:
 Associated pipelines will have to be flushed
thoroughly with on-spec naphtha
 Limitations would have to be imposed on the
scheduling of naphtha shipments.

Option 3. Water injection and mixing using remaining
tanks. Water can be sent to one of the other tanks
(T6217 A/B. The T6217C can be transferred to it. The advantages
from this option include:
 There is thorough mixing of SRN
 The mixing time will be less, as the SRN can mix
while it is filling the tank.

Conversely, the disadvantages
are:
 Only one tank will be available for
operation.
 If the procedure fails for any reason, then the
additional tank also contains contaminated naphtha.
 Associated pipelines will have to be flushed
thoroughly with on-spec naphtha
 Naphtha shipment schedules would be affected.

SOLUTION

Option 1 was selected as the
preferred method. As per the plan, 1% DW or 250 m3
of DW would be injected directly to the tank. The tank mixer
would be used to mix the SRN and DW, followed by tank settling
and draining of settled water. This procedure would be repeated
as required until the naphtha is completely washed and meets
all oxygenate specifications.

Successful water-washing plan.

Water injection to the tank started
on Feb. 26 during the day shift. Table 5 shows the result of
oxygenates, MeOH and chlorine content of the naphtha before and
after the water washing. Water draining started right after
nine hours of settling. After the first water wash, the
oxygenate level dropped to 60 ppm, close to spec, from the
average result of 240 ppm. Therefore, after the water was
drained, a second water-wash operation started on March 4,
after which the total oxygenates dropped to 40 ppm; both were
acceptable and on-spec. Fig. 8 shows how the oxygenate level
changed with water washing. Table 6 summarizes the moisture
content of the SRN before and after the water-washing
operations. The SRN then received a quality certificate and it
was successfully exported. The refinery continues to successfully
operate with all three naphtha tanks in service, and with no
further incidents of MeOH contamination. HP

Farzad
Ovaici received his MSc degree in chemical
engineering from Shiraz University in 1978. In 1979, he
began his career with Bandar Imam Petrochemical Co. in Iran. In
1980, he moved to the Isfahan refinery. Mr. Ovaici was
later responsible for reconstruction and
rehabilitation of Abadan refinery, a 630,000-bpd
refinery. This refinery severely damaged due to the
Iran-Iraq War. In 1992, he joined Tabriz Petrochemical
Co., and was assigned as project director for EB/SM,
and different polystyrene plants. Later, he was assigned
as chairman and managing director of Tabriz Petrochemical Co. In 2000, he
became the managing director of Kala Naft Canada Ltd. Mr.
Ovaici received an M. Sc. degree in engineering from the
Chemical and Petroleum Engineering School of University
of Calgary. He is a member of the Association of
Professional Engineers Geologists and Geophysicist of
Alberta Canada. In 2005, he moved from Canada to Oman and
joined Oman Refinery Co. as the general manager, of the
Mina Al-Fahal Refinery. Later, he was
promoted to general manager of the two refineries in Oman
Refineries and Petrochemical Co. Mr. Ovaici
joined Al-Ghurair Energy as the managing director, of refining and petrochemicals and is based in
Dubai, UAE. In addition to his position in Al-Ghurair
Energy, Mr. Ovaici is currently chief executive officer
of Libyan Emirates Oil Refining Co.

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